The expression of immune response (Ir) genes and the mechanism(s) of tolerance were explored in tetraparental (TP) mice. TP mice were prepared by fusion of C3H (H-2k, (T,G)-A--L low responders, a allotype) and (CWB x CKB) F1 (H-2k/b, (T,G)-A--L high responder b allotype) blastocysts followed by implantation in foster mothers. 10/19 offspring were chimeric as determined by H-2 typing of peripheral blood cells, spleen, thymus and lymph node cells. Mice were immunized with (T,G)-A--L and serum levels of anti-(T,G)-A--L antibodies and the allotype of the antibodies determined. Preliminary results indicate that a small portion of the TPs produce anti-(T,G)-A--L of low responder allotype. Samples from all TPs of lymph nodes, spleen, and thymus from all mice were frozen for H-2 analysis by immunofluorescence in order to correlate the distribution of parent cell types and the ability to make low and high responder (T,G)-A--L antibodies. The mechanism of tolerance involved in TPs was investigated by using both TP and parental cell types (i.e. C3H and CWB) as stimulators or responders in mixed lymphocyte cultures (MLC). These experiments showed that TP cells could serve as stimulator or responders for third party BALB/c cells. Generally TP cells did stimulate parental cell types but the TP cells were unresponsive to parental cells. The latter unresponsiveness was probably not due to the presence of MLC suppressor cells, since TP T cells were unable to affect positive MLC.